Wind turbines are a source of renewable energy that has appealed to our civilization for being able to compensate in part for the energies with a carbon origin or with a nuclear origin to produce electricity. Even if the amounts produced remain small, they cost nothing during production except the initial depreciation and maintenance, having no carbon impact and being able to work for 20 to 30 years, with nearly automatic control.
These wind turbines work in given wind speed ranges, as a function of their mechanical strength and of numerous other parameters linked to the wind class associated with the site according to standards such as that of the IEC (International Electrotechnical Commission).
When wind turbines are not working, the blades are feathered, i.e., they do not engage the wind when the wind is too strong, and they are in waiting position when the wind is insufficient, and the blades therefore remain almost immobile or free in rotation.
The shapes of the wind turbine blades are of the wing type, like an airplane wing with a pressure face, a blade back, a leading edge and a trailing edge, generating excess pressures and negative pressures of air. Now, the blades rotate at a high speed of rotation, between 10 and 20 revolutions/minute especially when they are at their maximum allowed yield, and the speeds reached at the end of the blades are very high, between 70 and 80 m/sec with therefore significant negative pressure or turbulence phenomena.
Now it is found that, under certain atmospheric conditions, the blades are covered with a layer of frost and at the very least even with a layer of ice, either during operation or when they are shut down, like any structure outdoors and especially when there is a strong exposure to bad weather.
Wind turbines are wisely installed in windy places that are the tops of hills or wind-swept mountains or at sea, and, offshore in particular, the conditions can be extreme.
If the blades become covered with frost, this poses numerous and serious problems of overloads, of imbalance, of vibrations, of breaking-off of sheets of ice or of frost.
If the blades are covered with a layer, even a thin layer, of frost or ice, it is very advantageous to be able to de-ice the blades to ensure the safe and continuous operation of the wind turbine. Actually, there may not be any wind with frost conditions that lead to the formation of a thick layer of frost; if the next day the wind conditions are suitable without icing conditions, despite a cold such that the thick layer of frost is still in place, it would be useful to de-ice to put the wind turbine in production condition.
Thus, certain wind farms that are already established and certain wind farms in the process of being set up or planned could find there a salvaging of a theoretical annual period of production of several percentage points, which is far from being negligible.
In addition, it is also known that the unwanted layer poses safety problems because it can cause the breaking of the blades or of the rotor. It is furthermore difficult to imagine very greatly increasing the mechanical strength of the blades to respond to this problem, which would adversely affect the operation during the rest of the time, i.e., the major part. This mechanical reinforcement will also have an impact on the manufacturing cost because it is, of course, the entire structure that has to be reinforced.
Another safety problem, that of the ice sheets that can form on the surface of the blades and that are separated from the surface to become true dangers for individuals brought to operate in the vicinity [sic].
The frost or the ice also causes, even a thin layer, an excess weight that adversely affects production and that also causes imbalances between the blades, leading to vibrations that are harmful to the operating life of the bearings of the rotor and of the different parts of the wind turbine.
Wind turbines are monitored and sensors are installed in the blades and the base of the rotor as well as an actuator to ensure the orientation of the blades, a mechanical unit known also as “pitch.” Therefore, it is noted that an electric power source already exists near the blades. Actually, it involves either a three-phase current for an electric actuator, or a 24-volt current for the end-of-travel indicators, etc., for a hydraulic actuator.
Wind turbine blades are made of composite materials having a base of polyester or epoxide matrices, with fiberglass reinforcements, carbon fibers, and with a core of lightweight material of the balsa or foam type.
Blades of large size are hollow, often made of two half-shells assembled in the median plane of the leading edges and trailing edges.
Solutions of the prior art are known whose purpose is de-icing wind turbine blades but which are unsatisfactory for numerous reasons.
For example, the patent application WO 2011018695 that describes segments of thermoelectric films having a base of polyimide-impregnated carbon can be cited. These films are added to the surface of the blades, which is not satisfactory with regard to the operating weather conditions and to the rapid degradation of this surface by the external damages of rain, hail, sleet.
Likewise, the patent application WO 201028653 discloses an embodiment of a layer of material that includes conductive nanoparticles but this requires the production of nanoparticles. The addition of metallic components adds to the equivalent radar signature, always liable to disrupt military and weather authorities.
The patent application WO 200079128 describes a heating system with an arrester comprising metal sheets that form resistance. This is a resistance heating system with the drawbacks linked to the fact of placing metal parts in the blades and of generating pinpoint sources of heat rather than a suitable distribution.
Wind farm operators are looking for a solution that is inexpensive, maintenance-free, lightweight for a perfect integration of the rotor, a balancing of the blades with a difference in weights with the same play of less than 5%, without danger to personnel, which does not change the profile of the blades, which does not complicate the production of the blades, which is esthetically unobtrusive, which avoids a modification of the wind turbine itself and which can be adapted to blades to be constructed but also for retrofitting on installed blades, the number of wind farms affected by frost already being very significant on all continents.